REVIEW: NEXUS-6647: Tell the user when NFC entry will expire

The log and describe output will contain _when_ the NFC is about to expire.

Issue
https://issues.sonatype.org/browse/NEXUS-6647

CI
http://bamboo.s/browse/NX-OSSF96
I think it would be simpler to use a `StringBuilder` and build up the message incrementally according to whether the expiration time is available or not, rather than create an interim detail string and shoehorn that into a pre-formatted message.